The historical reason why we wear costumes on Halloween

In addition to sacrificing animals to the gods and gathering around bonfires, Celts often wore costumes—probably animal skins—to confuse spirits, perhaps to avoid being possessed, according to the American Folklife Center at the U.S. Library of Congress.

By wearing masks or blackening their faces, Celts are also thought to have impersonated dead ancestors.

Young men may have dressed as women and vice versa, marking a temporary breakdown of normal social divisions.

In an early form of trick-or-treating, Celts costumed as spirits are believed to have gone from house to house engaging in silly acts in exchange for food and drink—a practice inspired perhaps by an earlier custom of leaving food and drink outdoors as offerings to supernatural beings.